For purposes of discussion, a pattern refers to a network of resistive elements on a substrate isolated from each other. The network includes pads, disks, rods and other types of resistive elements. An oxide layer is between the substrate and the network of resistive elements.
Polycrystalline silicon resistances are frequently formed in integrated circuits. The value of this resistance depends on the number of doping atoms added into the resistance during the integrated circuit fabrication process. One known method for locally inserting doping atoms is to perform a masking operation with photosensitive resin and introduce doping atoms by ionic implantation through the openings in the resin. However, this is a collective operation in the sense that all chips in a particular wafer and all wafers in a particular batch are made identical.
For some applications, it is useful to differentiate wafers from each other, or even chips from each other. For example, there may be a requirement to enter a specific reference, or more generally, there may be a requirement to write a specific program in a read only memory (ROM) of a single chip. A different photolithography mask can be made for each different inscription. The limit of this method is quickly reached due to the prohibitive cost of the different masks.
The method commonly used to perform these specific operations is to write directly onto a wafer using an electron beam. In other words, the resin is sensitive to electrons provided by an electron beam. The mask used becomes a virtual mask since it is written in the form of a program in an isolation machine. It is known that the wafer treatment rate of this type of isolation equipment using electron beams is low. Not only is it necessary to precisely describe the boundary of an element of the pattern to be created, which requires a very narrow beam, but the surface of every element of the pattern to be isolated has to be scanned, which would require a beam with a wider area to advance quickly.
Electron isolation machines are also very complex because the electron beam has to be positioned very precisely since the elements have small patterns, e.g., dimensions less than 0.1 μm. Electron isolation machines require large computer resources. The entire mask to be drawn has to be described in the form of a computer program.
Furthermore, the electron beam isolation method provides the resin surface with electrostatic loads that can deviate the electron beam. Thus, the shape of one element of a pattern creates electrostatic effects that influence the shape of the adjacent elements in the pattern. These proximity effects significantly complicate the write program used by the electron isolation machine that has to correct them. Furthermore, these proximity effects limit the minimum possible dimensions of the patterns.